AlMe₃ and ZnMe₂ Adducts of a Titanium Imido Methyl Cation:  A Combined Crystallographic, Spectroscopic, and DFT Study

A combined experimental and DFT study of the reactions of the titanium imido methyl cation [Ti(N^tBu)(Me₃[9]aneN₃)Me]⁺ (4⁺) with AlMe₃ and ZnMe₂ is described. Reaction of 4⁺ with AlMe₃ gave [Ti(N^tBu)(Me₃[9]aneN₃)(μ-Me)₂AlMe₂]⁺ (7⁺), the first structurally characterized AlMe₃ adduct of a transition metal alkyl cation and a model for the presumed resting state in MAO-activated olefin polymerizations. Reaction of 4⁺ with ZnMe₂ also gave a methyl-bridged heterobinuclear species, namely [Ti(μ-N^tBu)(Me₃[9]aneN₃)(μ-Me)₂ZnMe]⁺ (8⁺), the first directly observed ZnMe₂ adduct of a transition metal alkyl cation. At room temperature, all three metal-bound methyls of 8⁺ underwent rapid exchange with those of free ZnMe₂, whereas at 233 K only the terminal Zn−Me group exchanged significantly. Addition of AlMe₃ to 8⁺ quantitatively formed 7⁺ and ZnMe₂. Reaction of 4⁺ with Cp₂ZrMe₂ gave [Ti(N^tBu){Me₂(μ-CH₂)[9]aneN₃}(μ-CH₂)ZrCp₂]⁺ (10⁺) via a highly selective double C−H bond activation reaction in which both alkyl groups of Cp₂ZrMe₂ were lost. DFT calculations on models of 7⁺ confirmed the approximately square-based pyramidal geometries for the bridging methyl groups. Calculations on 8⁺ found that the formation of the Ti(μ-Me)₂Zn moiety is assisted by an N_imide→Zn dative bond. DFT calculations for the sterically less encumbered methyl cation [Ti(NMe)(H₃[9]aneN₃)Me]⁺ found strong thermodynamic preferences for adducts featuring N_imide→M (M = Al or Zn) interactions. This offers insight into recently observed structure−productivity trends in MAO-activated imido-based polymerization catalysts. Calculations on the metallocenium adducts [Cp₂Ti(μ-Me)₂AlMe₂]⁺ and [Cp₂Ti(μ-Me)₂ZnMe]⁺ are described, each showing α-agostic interactions for the bridging methyl groups. For these systems and the imido ones, the coordination of AlMe₃ to the corresponding monomethyl cation is ca. 30 kJ mol^-1 more favorable than for ZnMe₂.